The prior art has disclosed hydrostatic transmissions for mobile working machines in which a hydrostatic pump (primary unit) and at least one hydrostatic motor (secondary unit, are fluidically connected to one another by means of a closed hydraulic circuit. A primary unit may be situated with multiple secondary units in a closed hydraulic circuit, wherein the secondary units are arranged in parallel with respect to one another in terms of the circuit configuration. An internal combustion engine, for example a diesel engine of the mobile working machine, is coupled rotationally conjointly to the primary unit, and an output, for example an axle or a wheel of the mobile working machine, is coupled rotationally conjointly to the secondary unit. The mobile working machine thus has a traction drive which has a hydrostatic transmission.
The document EP 1 960 699 B1 discloses a hydrostatic transmission with which braking can also be performed. Here, in relation to traction operation, the power flows in the reversed direction from the output, via the secondary unit acting as a pump and via the primary unit acting as a motor, to the internal combustion engine, which is then driven in a passive cranking operating mode. The highly pressurized working line of the closed circuit is in this case safeguarded by means of a pressure-limiting valve, by means of which, too, a part of the braking power can be dissipated during the braking operation. In the case of said solution, however, the braking operation is triggered only when the driver explicitly expresses the demand by means of an actuation of the brake pedal.
The documents DE 10 2014 211 393 A1 and US 2014/0372000 A1 each disclose a hydrostatic transmission with which braking can also be performed. Here, it is sought to protect the internal combustion engine against overspeeding. For this purpose, a speed control system is described which identifies an overshooting of the braking power of the internal combustion engine and thereupon automatically triggers a high-power braking operation. A first part of the braking power is output to the internal combustion engine, whereas a second part of the braking power is converted by means of the pressure-limiting valve of the high-pressure line in question into heat.
It is furthermore known from the prior art to automatically initiate a braking operation by means of a hydrostatic transmission if the traveling speed of the mobile working machine in question becomes too high. Here, 2-position closed-loop controllers are used, such that the braking operation is activated if a maximum speed or activation threshold is overshot. A fixedly set braking torque then acts until the traveling speed falls below a deactivation threshold.
A disadvantage of such monitoring of the traveling speed is that the activation threshold and the deactivation threshold are fixedly set values. It may thus be the case, for example when the mobile working machine in question is traveling downhill, that the automatic braking is repeatedly initiated and ended again in an unpleasant and technically sub-optimum manner.
By contrast to this, it is the object of the disclosure to provide a hydrostatic transmission and a method with which these disadvantages are avoided.